This invention concerns lightweight fasteners such as for aerospace applications. A bolt with roll formed threads and better than usual effective thread run-out is provided. A shortened nut with a truncated thread for clearance from the bolt thread run-out is also provided. Because of such improvements the nut and bolt can each be one pitch length shorter than a conventional nut and bolt.
Weight of the fasteners is of great concern in airplanes and other aerospace applications. The nuts, bolts, rivets and the like employed for securing the structural elements of an airplane contribute a substantial portion to the total weight of the airplane since very large numbers of such fasteners are used. It is not unusual to use over 100,000 fasteners on one airplane. Thus, there has been a long effort to reduce the weight of fasteners without decreasing the strength, or preferably decreasing weight while increasing strength. Even an apparently small decrease in weight on an individual fastener can have a large impact on the total weight of an airplane.
Nuts and bolts are ubiquitous fasteners on aircraft. The vast majority of aircraft bolts have roll formed threads because of the superior fatigue properties of these threads as compared with machined threads. To make such bolts, a machined metal blank is rolled between a pair of thread forming dies for placing a thread on the blank. Metal is not removed from the blank in this process, but instead the metal is deformed as the thread forming die presses in to form the root of the thread. The displaced metal flows outwardly to form the crest of the thread. A conventional thread rolling die has uniform thread forming ridges and grooves over most of its width for making a uniform thread. The edges of the dies are, however, chamfered or rounded somewhat to avoid damage to the die. This chamfer results in a short run-out zone between the cylindrical shank of the bolt and the end on which threads are fully formed.
In conventional threaded aircraft fasteners the run-out zone has a length of up to two times the pitch of the thread. Within the run-out zone the root of the thread is not fully developed because of the chamfer on the roll forming die. That is, the thread is shallower than in the portion of the bolt where the thread is fully developed. Concomitantly, the crest of the thread in the run-out is not fully developed since less metal is displaced from the root. Thus, in the run-out the outside diameter of the crest of the thread is less than the major diameter in the fully threaded portion. The flanks of the bolt thread, which carry the tensile load on the nut and bolt combination, may or may not be fully developed in the run-out. Thus, in the run-out the thread is referred to as imperfect and nonfunctional. The maximum length of the run-out under the specifications used in the aerospace industry is 2P, where P is the pitch of the thread. The actual length of the run-out due to normal manufacturing variations is in the range of from 1.5P to 2P.
Nuts used with conventional aerospace fasteners typically have a counterbore collar concentric with the threaded hole through the nut. The length of the counterbore is such that when fully tightened, the threaded portion of the nut does not extend into the run-out on the bolt. If it were to extend into the run-out there would be thread interference between the crest of the nut thread and the incompletely formed bolt thread root, and the nut could not be properly tightened on the parts being secured.
The length of the cylindrical shank of a bolt from its head to the beginning of the thread run-out is referred to as the "maximum grip." If the fastener has a flush head, the grip is from the top of the head to the end of the shank. With a raised head not countersunk into one of the parts being secured, the grip is the cylindrical, unthreaded length of the shank. The plane corresponding to the maximum thickness of parts to be held by a bolt is sometimes also referred to as the "maximum grip." High strength aerospace fasteners are designed so that the maximum grip corresponds to the maximum thickness of the parts being secured together. The minimum grip is typically one-sixteenth inch (1.6 mm) less than the maximum grip. For example, an aircraft fastener may have a nominal length of one quarter inch (6.35 mm) for the maximum grip. The fastener would be used for securing together parts having a total thickness in the range of from three-sixteenths inch (4.76 mm) to one-quarter inch (6.35 mm).
The depth of the nut counterbore in a conventional aerospace fastener is the difference between the maximum grip and minimum grip plus about 1.5P. A small amount may be added to account for accumulated manufacturing tolerances. This means when the nut is secured against parts having the minimum grip, the ends of the threads in the nut are at the end of the run-out, a distance of about 1.5P from the end of the cylindrical shank of the bolt. When the nut is secured against parts having the maximum grip, the threads on the nut stops about one-sixteenth inch (1.6 mm) plus 1.5P from the end of the run-out. The same concept is present in metric bolts, and the difference between minimum grip and maximum grip is typically one millimeter or two millimeters. Thus, the depth of a nut counterbore is 1 mm or 2 mm plus 1.5P.
It has been recognized that if the thread run-out were reduced to 1P or less instead of about 2P, the length of the counterbore on the nut should be reduced, thereby reducing the total length of the nut. This also permits a shortening of the bolt thread and overall length by 1P. The weight savings in an airplane by reducing the length of both collar of the nut and bolt by as little as 1P can be quite substantial.
Aerospace bolts have been developed with an effective run-out of only about 1P. In one such design, for example, a special roll forming die is used. Instead of tapering the root of the thread and producing an imperfect thread, a full thread is carried to within 1P of the cylindrical shank. This has permitted reduction of the total length of the nut and bolt by 1P without significantly reducing the tensile strength of the nut and bolt combination.
This type of short run-out bolt has drastically reduced fatigue properties as compared with a conventional bolt having a run-out length of up to 2P. An exemplary tensile fatigue test simulates an application where the parts secured together do not have parallel faces. For example, two parts may be secured together with the face engaged by the nut being out of perpendicular with the axis of the bolt shank by three degrees.
In a three degree tensile fatigue test there is comparable off-axis loading which induces some bending in the bolt. In an exemplary fatigue test an assembly of a nut and bolt is cycled between an upper tensile load of 50% of the rated capacity of the combination and a minimum load of 5% of the rated load bearing capacity of the combination. The number of cycles to failure is measured.
In such a test the bolt with a modified thread having only an effective 1P run-out has only about 20% of the fatigue life of a conventional bolt with up to 2P run-out. For example, if a conventional bolt has fatigue life of 100,000 cycles, the modified lightweight bolt may have a fatigue life as low as 20,000 cycles in the three degree off-axis fatigue test. The adverse impact on tensile fatigue is apparently due to having a rather deep thread close to the maximum grip plane at the end of the cylindrical shank. Such a short run-out bolt is also substantially poorer in a lap shear test than a bolt with a run-out of 2P.
Thus, there are two types of shortcomings in existing aerospace fasteners. In one type of fastener there is adequate tensile strength and tensile fatigue strength, but the fastener is relatively long and therefore relatively heavy. On the other hand, a fastener has been produced which is shorter and hence lighter, but the tensile fatigue properties of that fastener are significantly adversely affected.
Thus, it is desirable to provide an aerospace fastener with roll formed threads where the length of the nut and bolt can be reduced without reducing either the tension capability of the combination or the off-axis tensile fatigue strength of the combination. It is desirable to make such a fastener using roll forming dies relatively unchanged from conventional roll forming dies. It is particularly desirable to provide such a combination of a nut and bolt that has increased strength as compared with conventional aerospace fasteners.